Which of the following organisms is known to attack the root system of plants?

Nematodes are microscopic, roundworms that are well-known for their ability to attack the root systems of various plants. These organisms can invade the roots directly, leading to various forms of damage that can stunt plant growth, reduce nutrient uptake, and make plants more susceptible to disease.

Root-knot nematodes, for example, create galls on the roots which disrupt the plant's ability to transport water and nutrients. They can also create openings for pathogenic fungi and bacteria to enter the plant, compounding the damage. Nematodes have a significant impact on agricultural productivity, particularly in crops where they are prevalent, thus making them a major concern for farmers and horticulturists.

In contrast, while insects, fungi, and bacteria can also be harmful to plants, they do not primarily target the root systems in the same manner as nematodes. Insects often feed on leaves or stems, whereas fungi primarily invade through above-ground plant tissues or through damaged areas. Bacteria can cause diseases that may affect root health, but they typically do not attack plants by infiltrating the roots in the same direct way that nematodes do. This focus on root systems is what sets nematodes apart in this context.